Switching DC-to-DC power converters having a multi-phase coupled inductor topology are described in U.S. Pat. No. 6,362,986 to Shultz, et al., the disclosure of which is incorporated herein by reference. These converters have advantages, including reduced ripple current in the inductors and switches allowing reduced per-phase inductance or reduced switching frequency, over converters having conventional multi-phase DC-to-DC converter topologies. As a result, converters with magnetically coupled inductors have a superior transient response without an efficiency penalty compared with conventional multiphase topologies. This allows a significant reduction (>50%) in output capacitance resulting in smaller, lower cost power solutions. The switching DC-to-DC converter described in U.S. Pat. No. 6,362,986 is not ideal for large voltage step-down or step up ratios and does not provide for isolation between input and output rails.
There are many applications for DC-to-DC power converters where isolation between input and output rails is desirable. For example, AC power supplies and AC adapters of many devices rectify household AC current to provide a high DC voltage, driving a DC-to-DC converter that drives a load. Many such devices have metal parts accessible to a user; user safety requires that there be a high impedance, or electrical isolation, between either side of the AC power connection and the output power rails.
In other applications, DC-to-DC converters may need to provide a large voltage step-down from input to output. This step-down ratio can be achieved by cascading multiple converter stages or by using a single stage having a transformer to step down the input voltage. Transformer-based topologies are also useful to step the voltage up.
Transformer based DC-to-DC power converters between input and output rails typically have a set of input switches that convert input DC to high frequency AC current, a transformer for providing voltage step-down and/or electrical isolation while magnetically coupling AC power, and a set of output switches or rectifiers for generating the DC output.
In some DC-to-DC converters, isolation is not required but transformers are still desired in power supplies because of a high step-up or step-down ratio of the voltages between input and output.
Many (not all) transformer based multiphase DC-to-DC buck-type or buck-derived power converters have output inductors. These inductors are typically not magnetically coupled to each other.